Starting with a clean slate

Einstein did not mince his words when referring to the conservative attitudes prevalent among natural philosophers of his time — '... in spite of great successes in particulars, dogmatic rigidity prevailed in matters of principles.' It took a great step

to break free from preconceived ideas about space and time, and to develop a chain of logic built on completely new foundations. The principles of symmetry emerged as a basic prerequisite of the laws of Nature. When asked 'How do you know, Professor Einstein, that your theories are right?', Einstein replied: 'If they are not right, then the Lord has missed a wonderful opportunity'

One such preconceived idea refers to measurement in general, and measurement of velocity in particular. Take the example of a passenger walking through a carriage of a moving train. His or her speed depends on the point of view. It will be quite slow as it appears to another passenger seated inside the carriage, faster as viewed by someone on the platform of a station as the train rushes past, and possibly faster still as viewed from outside the earth. We have an in-built prejudice that somewhere out in space there is an absolute point at rest from which one could determine the correct absolute motion, and also an absolute clock which keeps universal time.